Alkylphenols are materials of commerce desirable for their antioxidant properties. Many members of this class have commercial utility in such applications as antioxidants and stabilizing agents for fuel oils and antioxidants for food of diverse type. Among the phenols which are antioxidants the ortho-alkyl and ortho, ortho-dialkylphenols appear to be superior. That is to say, the ortho-alkylphenols and ortho, ortho-dialkylphenols seem to be better antioxidants than their isomers. There is a corresponding need to prepare such ortho-alkylated phenols with relatively high selectivity and yield.
The usual method of preparing alkylphenols is to alkylate phenols with an olefin, alkyl halide, or alcohol in the presence of an alkylating catalyst which generally is a Lewis acid. Catalysts which have been employed include strong inorganic acids (sulfuric acid, phosphoric acid, and hydrofluoric acid to name a few), strong organic acids (for example, sulfonic acids and cationic exchange resins bearing such acid functionalities), metal halides (boron trifluoride, aluminum halides, and zinc halides are exemplary) and inorganic oxides such as alumina and silica. A deficiency in all such methods is their limited selectivity for ortho-alkylation, that is, alkylation at available ortho positions occurs with only limited preference to alkylation at other available positions. Another limitation in such methods is that some 2,4-dialkylphenols undergo further alkylation to 2,4,6-trialkylphenols only with great difficulty, if at all. Still another disadvantage is the relatively high reaction temperature necessary where the more selective alkylating catalysts are used, for example, inorganic oxides.
Some instances of the rearrangement of alkyl phenyl ethers to the isomeric alkylphenol have been reported. For example, U.S. Pat. No. 2,289,886 discloses that alkyl phenyl ethers when treated with hydrogen fluoride afford both the isomeric alkylphenol and the dealkylated phenol. More recently U.S. Pat. No. 4,283,572 describes the rearrangement of nonyl phenyl ether to a mixture of phenol, monononylphenol, and dinonylphenol. The patentee in German Pat. No. 2,345,911 teaches the gas phase rearrangement of phenetole at 270.degree.-320.degree. C. over an alumina activated with sulfur trioxide, a strong Lewis acid, with the regioselectivity falling short of that observed in this invention. By "regioselectivity" is meant selectivity in the site of the aromatic ring to which the ether group migrates. Such sparse reports are in marked contrast to the well known thermal rearrangement of allyl phenyl ethers to allyl phenols (Claisen rearrangement) where the allyl group migrates selectively to an ortho or, less often, to a para position.
We have made the remarkable discovery that alkyl phenyl ethers undergo a thermal rearrangement in the presence of an alumina as catalyst to afford the isomeric ortho-alkylphenols with high yield and good selectivity. Not only is the thermal rearrangement of an alkyl phenyl ether to an alkylphenol as a general phenomenon without precedent, but the regioselectivity of the rearrangement to afford an ortho-alkylphenol is completely surprising.
Such a method of ortho-alkylating phenols has many advantages over the prior art methods. One advantage is formation of the ortho-alkylphenol at a substantially lower temperature than was previously possible. That is to say, the rearrangement occurs at a temperature lower than that necessary for alkylation of the phenol with, for example, an olefin using alumina as the alkylating catalyst. Since the alkyl phenyl ether may be prepared from a phenol under relatively mild conditions, our discovery makes possible a two-stage preparation of an alkylphenol via (1) formation of the alkyl phenyl ether followed by (2) rearrangement of the ether, both reactions proceeding under substantially milder conditions than direct alkylation of the phenol.
Another advantage of the invention described herein is its high regioselectivity in affording ortho-alkylated phenols. Thus, the prior art alkylating methods afford ortho-alkylated materials with varying selectivity, whereas the method we describe below affords ortho-alkylated products with substantially improved selectivity.
Still another advantage of the method which is our invention is that it affords products which sometimes are not otherwise readily available. For example, (2-alkylphenyl) alkyl ethers undergo rearrangement to the isomeric 2,6-dialkylphenol with great specificity, whereas direct alkylation of the corresponding 2-alkylphenol may fail to afford the desired 2,6-dialkylphenol, or do so only in relatively poor yield.
Yet another advantage of our method is the control it affords over the extent of alkylation. Rearrangement of secondary and tertiary alkyl phenyl ethers as described results in only the secondary or tertiary alkyl group being introduced into the aromatic nucleus. This is tantamount to monoalkylation of the parent phenol, where, contrastingly, traditional methods of alkylating phenols typically leads to polyalkylation.